Journal of Alloys and Compounds 537 (2012) 54–59 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom Preparation and optical characterization of Eu3+-doped CaTiO3 perovskite powders Duong Thi Mai Huong, Nguyen Hoang Nam, Le Van Vu, Nguyen Ngoc Long ⇑ Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received March 2012 Received in revised form 13 May 2012 Accepted 20 May 2012 Available online 29 May 2012 Keywords: CaTiO3:Eu3+ perovskite Sol–gel method Absorption Photoluminescence a b s t r a c t CaTiO3 perovskite powders doped with 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0 mol% Eu3+ were prepared by sol–gel technique followed by annealing at high temperatures The powders were characterized by X-ray diffraction, scanning electron microscopy, Raman scattering, absorption, and photoluminescence spectroscopy The obtained powders possessed orthorhombic crystal structure Raman spectra of the CaTiO3:Eu3+ powders exhibited seven new peaks at 798, 1048, 1188, 1371, 1441, 1601, and 1644 cmÀ1 which were assigned to the localized vibrational modes related to the complexes containing Eu3+ It was found that the band edge of the material shifted to the higher-energy side with increasing Eu3+-impurity content The photoluminescence of Eu3+ ions results from the radiative intra-configurational f–f transitions that happen between the 5DJ (J = 0, 1–3) exited states and the 7FJ (J = 0,1–4) ground states; the photoluminescence excitation of Eu3+ ions takes place from the 7F0 ground state to the 5DJ (J = 1–4), 5L6, and 5G2,6 exited states Ó 2012 Elsevier B.V All rights reserved Introduction Nanophosphors have been extensively investigated during the last decade because of their application potential for various high-performance and novel displays and devices The luminescence of rare-earth metal ions has a large technological importance in a variety of materials widely used in devices like phosphor lamps, displays, lasers and optical amplifiers The best host for these rare-earth ions is inorganic materials like Y3Al5O12, Y2O3, YVO4, LaF3, CaTiO3, LaPO4 etc [1,2] because these ions generally show high quantum yields in the above hosts Calcium titanate (CaTiO3) perovskite phosphor has attracted considerable attention and represents one of the most important classes of mixed oxides CaTiO3 doped with rare-earth presents various applications in the field of optoelectronic devices Recently, rare-earth doped CaTiO3 has attracted significant attention because of its strong luminescence properties, good chemical stability and promising applications in field emission displays [3] and white light-emitting diodes [4] However, to the best of our knowledge, most of studies [3,5–10] were focused on long afterglow phosphorescent materials, for example, CaTiO3 doped with praseodymium (Pr3+) ions Europium (Eu3+) ion is one of the most popular and important rare-earth dopants because Eu3+-doped phosphors are well known to be promising materials for electroluminescent devices, optical amplifiers, and lasers In the existing literature, there are few studies devoted to Eu3+ doped CaTiO3 (CaTiO3:Eu3+) [11–13] ⇑ Corresponding author E-mail address: longnn@vnu.edu.vn (N.N Long) 0925-8388/$ - see front matter Ó 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.jallcom.2012.05.087 CaTiO3 was synthesized by various methods: high temperature solid state reaction [6], co-precipitation [8], spray pyrolysis [9], sol–gel [10] and microwave assisted hydrothermal method [13] Among the above mentioned methods, sol–gel is the simple and widely used one for preparation of CaTiO3 In the present paper, we report on CaTiO3:Eu3+ powders prepared by sol–gel technique followed by heating at high temperatures The powders were characterized by X-ray diffraction, scanning electron microscopy, Raman scattering, absorption, and photoluminescence spectroscopy It was found that the photoluminescence (PL) of Eu3+ ions results from the radiative intra-configurational f–f transitions that happen between the 5DJ (J = 0,1–3) exited states and the 7FJ (J = 0,1–4) ground states; the photoluminescence excitation (PLE) of Eu3+ ions takes place from the 7F0 ground state to the 5DJ (J = 1–4), 5L6, and 5G2,6 exited states It was noted that the photoluminescence intensity was strongest in the samples doped with 3.0 mol% Eu3+ Experimental Ca1ÀxEuxTiO3 with x = 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0% mol ratio of Eu3+ ions powders were synthesized by sol–gel method using the following precursors: CaCl2, TiCl4, and Eu(NO3)3 All chemicals are of analytic grade without further purification A mixed aqueous solution contained the above chemicals with the appropriate mol ratio Ca: Eu: Ti = (1Àx): x: was prepared The mixture was then constantly stirred to get an opalescent solution Citric acid (CA) was dissolved in the double distilled water to form a 50% CA solution The CA solution was added into the opalescent mixture under constant magnetic stirring at 90 °C After h of stirring, the sol changed into a yellow chrome homogeneous gel The gel was dried at 120 °C for 24 h to remove the water, and was then annealed at 300 °C for 30 After that, an ash-gray powdered product was obtained In order to support the crystallization D.T.M Huong et al / Journal of Alloys and Compounds 537 (2012) 54–59 of materials, the resultant product was annealed at different temperatures ranging from 700 to 1000 °C in air for or h After the heat treatment powdered materials had white color Crystal structure of the powders was analyzed by X-ray diffraction (XRD) using an X-ray diffractometer SIEMENS D5005, Bruker, Germany with Cu Ka1 (k = 0.154056 nm) irradiation The surface morphology of the samples was observed by using a JSM 5410 LV, JEOL, Japan scanning electron microscope (SEM) The composition of the samples was determined by an energy-dispersive X-ray spectrometer (EDS) OXFORD ISIS 300 attached to the JEOL-JSM5410 LV scanning electron microscope Raman scatting spectra measurements were carried out by using LabRam HR800, Horiba spectrometer with 632.8 nm excitation Diffuse reflection spectroscopy measurements were carried out on a VARIAN UV–VIS–NIR Cary5000 spectrophotometer The spectra were recorded at room temperature in the wavelength region of 200–900 nm Absorption spectra of the samples were obtained from the diffuse reflectance data by using the Kubelka–Munk function [14]: FRị ẳ Rị2 K ẳ 2R S ð1Þ where R, K and S are the reflection, the absorption and the scattering coefficient, respectively The PL and the PLE spectra measured at room temperature were carried out on a spectrofluorometer Fluorolog FL 3-22 Jobin–Yvon–Spex, USA with a 450 W xenon lamp as an excitation source Results and discussion 3.1 Structure characterization and morphology Fig 1(a) shows XRD patterns of the powders CaTiO3 doped with 3.0 mol% Eu3+ annealed at different temperatures ranging from 300 to 1000 °C for h As can be seen from the figure, the samples annealed at 300 °C exhibited a bad crystallinity: The characteristic Fig XRD patterns of (a) the powders CaTiO3 doped with 3.0 mol% Eu3+ annealed at different temperatures ranging from 300 to 1000 °C for h, (b) the powders CaTiO3 undoped and doped with 1.5, 2.0, 3.0, and 5.0 mol% Eu3+ annealed at 1000 °C for h in air 55 peaks of CaTiO3 appeared with very weak intensity The samples exhibited better cystallinity with increasing annealing temperature At calcinating temperatures of 800, 900, and 1000 °C the samples displayed a good crystallization XRD patterns of the powders CaTiO3 undoped and doped with 1.5, 2.0, 3.0, and 5.0 mol% Eu3+ annealed at 1000 °C for h in air are shown in Fig 1(b) All the peaks in the XRD patterns clearly indicate that the CaTiO3:Eu3+ samples possess orthorhombic crystal structure No other diffraction peaks are detected except for the CaTiO3 related peaks The lattice constants determined from the XRD patterns are a = 5.432 Å, b = 7.643 Å and c = 5.390 Å, which are in good agreement with the standard values (a = 5.440 Å, b = 7.643 Å and c = 5.381 Å, JCPDS card No 22-0153) The average size of the crystallites was estimated by Debye–Scherrer’s formula [15]: D¼ b 0:9k cos h ð2Þ where b is the full width at half maximum (FWHM) in radians of the diffraction peaks, h is the Bragg’s diffraction angle and k = 0.154056 nm The calculated size of the CaTiO3 nanocrystallites was estimated to be 24 nm Typical SEM images of CaTiO3 powders undoped and doped with 3.0 mol% Eu3+ calcined at 1000 °C for h under atmospheric condition are shown in Fig From the figure it can be noted that the fine crystallites agglomerated into big slabs with the size of several micrometers Representative EDS spectra of the CaTiO3 powders are shown in Fig The EDS spectrum of the CaTiO3 sample doped with 5.0 mol% Eu3+ exhibits the peaks related to element Eu It is noted that the gold peaks observed in the EDS spectra originated from the gold Fig Typical SEM images of CaTiO3 (a) undoped and (b) doped with 3.0 mol% Eu3+ calcined at 1000 °C for h under atmospheric condition 56 D.T.M Huong et al / Journal of Alloys and Compounds 537 (2012) 54–59 divided into two regions In the low-energy region we found 10 peaks at 154, 177, 222, 245, 287, 337, 471, 494, 530, and 640 cmÀ1, which are in good agreement with the existing literature and are usually attributed to the Raman modes of the orthorhombic crystal structure CaTiO3 [18–24] The peak at 154 cmÀ1 is related to the CaTiO3 lattice mode The peaks at 177, 222, 245, 287, and 337 cmÀ1 are assigned to O–Ti–O bending mode The peaks at 471, 494, and 530 cmÀ1 are related to Ti–O3 torsional modes and the 640 cmÀ1 peak is characteristic of Ti–O symmetric stretching mode It is interesting to note that when europium was introduced into the CaTiO3 powders, in the high-energy region we first time observed seven new peaks at 798, 1048, 1188, 1371, 1441, 1601, and 1644 cmÀ1 (lines b–e in Fig 4) These vibrational modes may be related to LVMs of the Eu3+-containing complexes with different configurations 3.3 Absorption and photoluminescence spectra Fig The EDS spectra of the undoped CaTiO3 and Eu3+-doped CaTiO3 powders with 5.0 mol% Eu3+ layer deposited on silicon substrate for enhancement of conductivity in the EDS measurement It is known that effective radii of Ca2+, Eu3+, and Ti4+ ions in octahedral sites are 1.00, 0.947, and 0.605 Å, respectively [16] It is expected that the Eu3+ ions can substitute for the Ca2+ ions more easily than for the Ti4+ ions in CaTiO3:Eu3+ lattice because ionic radii for Ca2+ and Eu3+ are close In addition, Mazzo et al [13] showed a simulated orthorhombic lattice of CaTiO3:Eu3+, which illustrated the substitution of Eu3+ ions for Ca2+ ions in octahedral sites 3.2 Raman scattering spectra Raman spectroscopy is an important and useful tool for obtaining information about the vibrational modes of the materials and it is well known that a small concentration of impurities introduced into a perfect crystal will have little effect on vibrational modes, in some case even there may appear vibrational modes lying outside of the allowed frequency range of the perfect crystal [17] These are called localized vibrational modes (LVMs) Typical room temperature Raman spectra of the undoped CaTiO3 and the CaTiO3:Eu3+ powders with various contents of Eu are shown in Fig As seen from the figure, the spectra can be Fig Typical Raman spectra of the CaTiO3 powders undoped and doped with 1.0, 2.0, 3.0, and 5.0 mol% Eu3+ Fig depicts diffuse reflection spectra measured at room temperature of the undoped CaTiO3 and the Eu3+-doped CaTiO3 powders with various dopant contents Can be seen that in addition to the strong absorption in the energy region higher than 3.75 eV, four weak absorption peaks located at 3.12 eV (397 nm), 2.66 eV (466 nm), 2.07 eV (599 nm), and 1.92 eV (646 nm) were clearly observed from the reflection spectra of the 2.0, 3.0 and 5.0 mol% Eu-doped CaTiO3 samples, in which two absorption peaks at 3.12 and 2.66 eV were found by previous work [13] These four absorption peaks can be assigned to the transitions 7F0 ? 5L6, 5D2, D0, and 7F3 ? 5D0 of the Eu3+ ion, respectively, because their energies are in good agreement with those of the basic and excited states of the Eu3+ ion [2] Absorption spectra of the CaTiO3:Eu3+ samples obtained from the diffuse reflectance data by using the Kubelka–Munk function F(R) are shown in Fig All the spectra exhibit a sharp absorption edge and an onset of absorption at 3.5–3.6 eV The inset of Fig obviously shows the mentioned above four of absorption peaks related to the optical transitions within Eu3+ ion in the CaTiO3 samples doped with 2.0, 3.0, and 5.0 mol% Eu3+ It is known that the band structure of the CaTiO3 displays a direct band gap at C point [25] The relation between the absorption coefficients (a) and the incident photon energy (hm) for the case of allowed direct transition is written as follows [26]: Fig Diffuse reflection spectra at room temperature of the undoped CaTiO3 and the Eu3+-doped CaTiO3 powders Four absorption peaks related to the optical transitions within Eu3+ ion are clearly observed in the spectra of the 2.0, 3.0, and 5.0 mol% Eu3+-doped CaTiO3 samples 57 D.T.M Huong et al / Journal of Alloys and Compounds 537 (2012) 54–59    h ð3p2 nÞ2=3 DEg ẳ 2meh 4ị where  h is the reduced Planck’s constant, mÃeh is the reduced effective mass of electron and hole (m1 ẳ m1 ỵ m1 ; me and mÃh are the eh e h where A is a constant and Eg is the band gap of the material The plots of ẵFRị hm2 versus hm for the undoped and the Eu3+-doped CaTiO3 powders are represented in Fig By extrapolating the straight portion of the graph on hm axis at a = 0, we found the band gaps of the CaTiO3 powders doped with the concentration of 0, 1.5, 2.0, 3.0, and 5.0 mol% Eu3+ to be 3.670, 3.670, 3.687, 3.695, and 3.719 eV, respectively Thus, with increasing Eu3+-dopant content from to 5.0 mol%, the optical band gap is gradually increased from 3.670 to 3.719 eV The similar phenomenon was also observed for ZnO doped with any of the group III elements (B, Al, Ga, In) and for many various semiconductors (see, for example, Ref [27]) This phenomenon can be explained as follows When the Eu impurity atoms of valence substitute for the Ca atoms of valence in CaTiO3:Eu3+ lattice, the Eu atoms become donors, which can give up conduction electrons If we introduce a lot of the Eu donors, the conduction electron concentration is increased; the Fermi level will rise more and more towards the conduction band Since the states below the Fermi level are already filled, according to the Pauli Exclusion Principle, the fundamental transitions to the states below the Fermi level are forbidden; hence the optical absorption edge should shift to higher energy side This is the well-known Burstein–Moss effect [28–30] According to the Burstein–Moss effect, the broadening of the optical band gap DEg is: effective masses of electron and hole, respectively), and n is the carrier concentration Therefore, the increase of Eu3+ impurity content making carrier concentration increase, leads to the high-energy shift of the band gap, as observed in our experiment In order to confirm the mentioned explanation, we measured the resistivity of some CaTiO3 samples with various Eu3+ contents The powders were pressed in tablet form with the size of 0.5 mm in thickness and 0.6 cm in diameter by a pressure of 4.3  108 Pa The impedance of the tablets was measured at room temperature The resistivities (q) were found to be >5.0  108, 2.0  108, and 8.5  107 X cm for undoped, mol% Eu3+, and mol% Eu3+ doped CaTiO3 powders, respectively Thus, with increasing the Eu3+ dopant content the conduction electron concentration increases, which decreases the samples resistivity Fig shows the room temperature PL spectra under excitation wavelength of 398 nm of CaTiO3 powders doped with various concentrations of Eu3+ It is noted from the inset of the Fig that the PL intensity was strongest in the samples doped with 3.0 mol% Eu3+ When increasing Eu3+ concentration higher than 3.0 mol% the PL intensity decreased Recently, Fu et al reported that the optimal concentrations for obtaining the highest PL intensity of CaTiO3:Eu3+ were 28 mol% of Eu3+ in the samples prepared by solid-state reaction [11] and 16 mol% of Eu3+ in those prepared by sol–gel method [12], while Mazzo et al reported that the optimal concentration of Eu3+ is mol% [13] When the concentration of an activator is higher than an appropriate value, the luminescence of the phosphor is usually lowered This effect is called concentration quenching The origin of this effect is known to be one of the following: the cross-relaxation between the activators, excitation energy migration to quenching centers or the surface states acting as quenching centers, the pairing or coagulation of activator ions and their change to quenching centers As mentioned above, the concentration quenching occurs at different concentrations maybe because the samples were prepared by different methods In fact, under various technological conditions Eu3+ ions were differently incorporated into the samples In order to interpret the origin of the emission lines, the room temperature PL spectrum under 398 nm excitation wavelength of CaTiO3 powder doped with 3.0 mol% of Eu3+ is illustrated in Fig The groups of emission lines located in the range of wavelength from 590 to 725 nm are attributed to the radiative transitions from Fig The plots of ½FðRÞ Â hm2 Š versus photon energy hm for the undoped CaTiO3 and the CaTiO3 powders doped with 1.5, 2.0, 3.0, and 5.0 mol% Eu3+ Fig Room temperature PL spectra under excitation wavelength of 398 nm of CaTiO3 powders doped with various concentrations of Eu3+ Fig Plots of Kubelka–Munk F(R) versus photon energy hm for the undoped CaTiO3 and the Eu3+-doped CaTiO3 powders The inset shows four absorption peaks related to the optical transitions within Eu3+ ion in the spectra of the 2.0, 3.0, and 5.0 mol% Eu3+-doped CaTiO3 samples ahm ẳ Ahm Eg ị1=2 3ị 58 D.T.M Huong et al / Journal of Alloys and Compounds 537 (2012) 54–59 Fig Room temperature PL spectrum under 398 nm excitation wavelength of CaTiO3 powder doped with 3.0 mol% of Eu3+ Fig 11 Energy level diagram of Eu3+ ions and the observed excitation and emission transitions Fig 10 Typical PLE spectrum monitored at 615 nm emission line of CaTiO3:3.0 mol% Eu3+ powders the D0 exited states to the FJ (J = 1–4) ground states, namely, the groups of lines at 592, 615, 654, and 695 nm are assigned to the emission transitions from the 5D0 excited state to the 7F1, 7F2, 7F3, and 7F4 ground states, respectively Some groups of very weak emission lines at 430, 447, 465, 489, 511, 527, 540, 555, and 580 nm are assigned to 5D3 ? 7F0, 7F2; 5D2 ? 7F0, 7F2, 7F3; 5D1 ? 7F0, 7F1, 7F2; and 5D0 ? 7F0 transitions, respectively (the inset of Fig 9) It is worth noting that all the emission line groups have the same excitation spectra, which prove that all these lines possess the same origin Typical PLE spectrum monitored at 615 nm emission line of CaTiO3:3.0 mol% Eu3+ powders is depicted in Fig 10 The groups of excitation lines located around 362, 376, 398, 418, 465, and 526 nm are attributed to the absorption transitions from the 7F0 ground state to the 5D4, 5G2,6, 5L6, 5D3, 5D2, and 5D1 excited states, respectively Fig 11 shows the energy level diagram of Eu3+ ions and the observed excitation and emission transitions in f–f configuration of Eu3+ ions Finally, it is noted that contrary to Pr-doped CaTiO3 powders, our CaTiO3:Eu3+ samples not exhibit a long afterglow luminescence The afterglow luminescence (phosphorescence) occurs due to the thermally stimulated recombination of trapped charged carriers Fig 12 depicts the decay behavior of the 615 nm (5D0 ? 7F2 transition) emission line for Eu3+ in the CaTiO3:3.0 mol% Eu3+ samples As seen from the figure that the experimental data were very well fitted using a double-exponential function: Fig 12 Decay curve of the 615 nm (5D0 ? 7F2 transition) emission line for Eu3+ in the CaTiO3:3.0 mol% Eu3+ samples Itị ẳ A1 expt=s1 ị ỵ A2 expt=s2 ị ð5Þ where I(t) is the phosphorescence intensity, A1 and A2 are the constants, and s1 and s2 are the decay constants (or lifetimes) The results showed that two lifetimes, a fast one s1 = 0.194 ms, and a slow one s2 = 0.919 ms have been observed for the 5D0 ? 7F2 emission of Eu3+ The fact that our CaTiO3:Eu3+ samples not exhibit a long afterglow luminescence indicated there are not the metastable traps in these samples Conclusion CaTiO3:Eu3+ perovskite powders were synthesized by sol–gel method followed by annealing at high temperatures At calcinating temperatures higher than 800 °C the samples displayed a good crystallization The obtained powders possess orthorhombic crystal structure with lattice constants a = 5.432 Å, b = 7.643 Å and c = 5.390 Å The average sizes of the crystallites estimated by Debye–Scherrer’s formula are 24 nm Raman scattering spectra show new peaks observed at 798, 1048, 1188, 1371, 1441, 1601, and 1644 cmÀ1 These vibrational modes may be related to LVMs of the complexes containing Eu3+ with different configurations With increasing Eu3+-dopant content from to 5.0 mol%, the optical band gap is gradually increased from 3.670 to 3.719 eV, which is D.T.M Huong et al / Journal of Alloys and Compounds 537 (2012) 54–59 assigned to Burstein–Moss effect The photoluminescence intensity is strongest in the samples doped with 3.0 mol% Eu3+ The PL of Eu3+ ions results from the radiative intra-configurational f–f transitions that happen between the 5DJ (J = 0, 1–3) exited states and the FJ (J = 0,1–4) ground states; the PLE of Eu3+ ions takes place from the 7F0 ground state to the 5DJ (J = 1–4), 5L6, and 5G2,6 exited states Two lifetimes s1 = 0.194 ms, and s2 = 0.919 ms have been observed for the 5D0 ? 7F2 emission of Eu3+ Acknowledgments This work is supported in part by the Grant-in-Aid for Scientific Research from Ministry of Science and Technology of Vietnam (NAFOSTED, Project No 103.02.51.09) Authors thank Dr Tran Thi Kim Chi for the decay time measurement [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] References [24] [1] H Chander, Proceedings of ASID, 8–12 October 2006, New Delhi, p 11 [2] J.W Stouwdam, Lanthanide-doped nanoparticles as the active optical medium in polymer-based devices, Doctoral Thesis, Twente University, Netherlands, 2004 [3] X Liu, P Jia, J Lin, G Li, J Appl Phys 99 (2006) 124902 [4] T Jüstel, H Nikol, C Ronda, Angew Chem Int Ed 37 (1998) 3084 [5] E Pinel, P Boutinaud, G Bertrand, C Caperaa, J Cellier, R Mahiou, J Alloys Compd 374 (2004) 202 [6] M Li, R.G Jin, R Yang, Y.F Li, J.X Liu, Adv Mater Res 11–12 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T Ikegami, K Ebihara, Thin Solid Films 513 (2006) 90  ... undoped CaTiO3 and the CaTiO3 powders doped with 1.5, 2.0, 3. 0, and 5.0 mol% Eu3 + Fig Room temperature PL spectra under excitation wavelength of 39 8 nm of CaTiO3 powders doped with various concentrations... in CaTiO3 :Eu3 + lattice because ionic radii for Ca2+ and Eu3 + are close In addition, Mazzo et al [ 13] showed a simulated orthorhombic lattice of CaTiO3 :Eu3 +, which illustrated the substitution of. .. spectra at room temperature of the undoped CaTiO3 and the Eu3 +-doped CaTiO3 powders Four absorption peaks related to the optical transitions within Eu3 + ion are clearly observed in the spectra of